Embedded Niobium Using PI-2611 for Superconducting Flexible Cables
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Embedded Niobium Using PI-2611 for Superconducting Flexible Cables Simin Zou1, Yang Cao1, George A. Hernandez1, Rujun Bai1, Vaibhav Gupta1, John A. Sellers1, Charles D. Ellis1, David B. Tuckerman2 and Michael C. Hamilton1* 1 Alabama Micro/Nano Science and Technology Center, Electrical and Computer Engineering Department, Auburn University, Auburn, AL, 36849, USA. 2 Microsoft Research, Redmond, WA, 98052, USA. * Corresponding Author E-mail: [email protected], Tel: 334-844-1879 ABSTRACT Dense, controlled-impedance, superconducting cables with small cross-sections are desirable, especially for quantum computing applications. In this study, superconductivity properties, rf microwave response and mechanical reliability performance of embedded Nb dc cables and Nb microstrip transmission line resonators with different thicknesses of polyimide PI2611 encapsulation layers (0, 4 and 8 μm) have been investigated. Critical temperature (Tc) and critical current (Ic) of embedded Nb dc cables are ~ 8.2 K and ~ 0.2 A, respectively. Embedded Nb resonators yield high loaded quality factor (QL), with values as high as 14481 at ~ 1.2 K and at a fundamental resonance of ~ 2 GHz. From mechanical fatigue testing, we have observed that a polyimide encapsulation layer can effectively enhance the mechanical reliability of superconducting Nb flexible cables. INTRODUCTION One of the major limitations to constructing densely-integrated, cryogenic electronic systems is the electrical interconnect technology [1-2]. Superconducting thin-film cables are expected to be more practical in cost and physical size and provide higher performance for signal transmission with low thermal leakage, compared to semi-rigid coaxial cables that are currently being used. Promising signal density and flexibility (i.e., bend radius) have been reported for non-embedded superconducting flexible cables in [3-4]. Furthermore, building superconducting flexible cables using an embedded structure is attractive since it has been observed to increase reliability and is necessary for cables in configurations that can provide reduced cross-talk, such as stripline. This technology is expected to be beneficial to multiple cryogenic electronics applications, including cryogenic quantum computers [5] and low-noise superconducting parametric amplifiers [6]. However, material selection, fabrication limitations, performance and reliability are all issues that must be considered. In this paper, we explore fabrication and performance trade-offs related to narrow Nb superconductor lines embedded in polyimide. A low loss polymer dielectric (polyimide PI-2611) [7] was chosen for the encapsulation layer, as well as for the cable substrate. A low temperature polyimide cure process was developed in order to protect the Nb superconductivity during subsequent fabrication processes. We found that embedded Nb superconducting cables show very small transmission line loss and enhanced mechanical reliability. More importantly, this type of embedded structure can potentially be applied to build mor
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